The present invention relates to connecting elements (joints) allowing for angular displacements between mechanical components (links).
Components of mechanical systems are often required to be movable relative to each other in angular (rotational) motion about three intersecting mutually perpendicular axes while maintaining their relative distances in translational directions.
Examples of such applications are steering systems of surface vehicles (cars and trucks), wrists of robotic devices, connections between tension-compression struts and base and moving platform in so-called xe2x80x9cparallel kinematicsxe2x80x9d machine tools (Stewart Platform designs, Hectapods), etc.
Usually, such connections between mechanical components are realized by spherical (ball) joints. A typical ball joint (see also Prior Art, FIG. 1) has an accurately fabricated steel ball attached to one of the connected mechanical components. The ball is fit into precision socket (spherical cavity of essentially the same radius) attached to the other connected mechanical component. The socket is specially designed to realize a captive engagement with the ball. Since the connection must withstand tension/compression loads between the connected components, contact between the ball and the socket should be lubricated and designed as a wear and friction-resistant pair, usually the hardened and ground steel ball riding against polished bronze surface of the socket. Attachment in a precision manner of one component connected by the joint to the hardened steel ball requires expensive manufacturing operations. Fabrication of the precision spherical cavity and the multi-part design of the socket to achieve the captivity effect can also be expensive. The device must be sealed against leakage of the lubricant and against contamination. Since some space between the ball and the socket is required to accommodate the lubricant, such connection always exhibits backlash for relative translational movements of the connected components. Presence of the backlash is usually undesirable but its elimination by applying preload would greatly increase friction in the joint and would complicate the design; it also requires even more precision fitting in order to reduce the clearance between the ball and the socket.
Thus, the prior art is represented by an expensive design requiring high-quality materials and high-precision fabrication. In the same time, the prior art ball joint has high friction even with lubrication, especially at high translational loads transmitted through the connection and at the reversal points of the relative motions between the ball and the socket since at these points there are no motions and thus no hydrodynamic effect. As a result, these points are characterized by boundary lubrication conditions and thus, high friction. When high tension/compression forces are acting between the connected links, these forces are acting as normal forces in the contact between the ball and the socket, thus generating high friction forces not desirable for functioning of the joint. Since the ball and the socket are frictionally connected, their relative motion begins only after the driving torque/moment exceeds the moment due to the static friction. As a result, conventional ball joints are not responding to input driving torques of small magnitudes.
The costs of conventional ball joints are high due to the required high precision and further increase due to the need for precision assembly of several parts of the socket (in order to achieve the captive effect) and for providing lubrication and sealing systems.
The present invention addresses the inadequacies of the prior art by providing a three-degrees-of-freedom rotational joint which retains the kinematic structure of the prior art spherical joint but does not have its design shortcomings. The preferred embodiment of the proposed joint comprises at least one high shape factor (thin-layered) elastomeric element whose inner and outer surfaces are concentric spherical surfaces with the center coinciding with the intersection point of the three rotational axes of the joint. The elastomeric element is preloaded in compression. The shape factor is commonly defined as ratio of the loaded surface area to the total free side surface areas. The inner (concave) and the outer (convex) surfaces of the elastomeric element are supported by rigid convex and concave spherical surfaces, respectively, attached to the mechanical links connected by the joint. In this design, relative angular displacements (rotations) between the connected mechanical links are accommodated by shear deformations in the thin-layered elastomeric elements. In the same time, translational forces between the connected mechanical components and the preload forces are accommodated by compression of the thin-layered elastomeric elements. It is known that large shear deformations are realizable (up to and sometimes exceeding 100-150% of thickness of the elastomeric element), while compression stiffness is at least 1-5,000 times greater than the shear stiffness. The compression deformations are much smaller than shear deformations, and very high specific compression loads can be accommodated, up to and exceeding 650 MPa (100,000 psi). Since these loads are way above the specific pressures allowable for frictional connections, the size of the proposed joint can be reduced for a given rated load.
Since the mobility in the proposed joint is due to internal shear deformations, no lubrication or sealing is required. Since it is known that the shear resistance is not significantly influenced by compression forces, the preload does not increase the resistance to angular motions while completely eliminating the undesirable backlash. The preload also keeps the convex (convex spherical member) and the concave (socket) rigid spherical surfaces in a force-locked condition for any directions of the translational forces (compression or tension), thus the captive design of the prior art ball joint is not needed and the complexity of the unit is reduced. Since there is no direct contact between the spherical member and the inside surface of the socket, both can be made from a variety of materials including inexpensive metals without heat treatment, plastics, etc., and no expensive surface finishing is required.
Use of internal shear deformation instead of sliding friction makes the joint sensitive to input torques/moments of any magnitude, even very small.